The proposed work investigates the relative roles of body temperature and endogenous circannual rhythms in controlling reproductive maturation in hibernating arctic ground squirrels. Specific goals include: (1) assessing the temperature sensitivity of reproductive hormone secretion in hypothermic and active animals, (2) determining the role of periodic arousals in testis growth, (3) characterizing secretion and metabolic clearance rate of gonadotropins and testosterone in animals naturally at body temperatures of 2 degrees Centigrade and 37 degrees Centigrade, (4) describing the natural pattern of testis growth and development over the hibernation season and testing the influence of elevated temperature on the timing of this pattern, (5) assessing the thermoregulatory options open to hibernating animals to avoid temperature constraints on endocrine function, and (6) determining the importance of sustained normothermic body temperature for sperm viability. These experiments should elucidate adaptations for function of basic regulatory mechanisms controlling reproduction and fertility under extremes of body temperature; they should help determine the scope of endocrine responsiveness in mammals, especially the temperature limit below which hormone secretion or turnover cannot occur.